Lipstick Jungle, which is based on the writer's best-selling book, follows the trials and tribulations of three best friends in their 40s.
The show will be made as a pilot for NBC and, if given the green light, it would appear on the network's autumn schedule.
Gershon has been cast as Wendy Healy, the powerful chief of a movie studio.
Bushnell tells the New York Post, "She's an actress whose work I've admired for a long time. She lives in New York, and I'm thrilled that she's our first on board."
Like Sex and the City, Lipstick Jungle will be set in Manhattan, but with older, more successful female characters.
The two remaining leading women, fashion designer Victory Ford and magazine editor Nico O'neilly, have yet to be cast.
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